Laser printers typically operate by using a D-roller to "grab" a sheet of paper (or other transfer medium) from a paper tray, and to send the paper on its way through a processing path within the printer. After initiating the process, a leading edge sensor detects the leading edge of the paper to allow for monitoring of the printing process. As timing is essential in electrophotographic (EP) processing, it is important to know where the paper is throughout the process and when it should be at specified stations. The leading edge sensor and other sensors in the system provide a means for monitoring the process.
Within the printer, a laser beam is scanned across an electrically charged photoconductor surface, such as a drum or belt, to form a latent image on the surface. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to the laser beam. A visible image is developed by toners which are selectively attracted to the photoconductor surface, either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode, and the toner. The photoconductor may be either positively or negatively charged, and the toner similarly may contain negatively or positively charged particles. For receiving the image to be printed, the paper is given an electrostatic charge and passed close to a photoconductor surface. As the paper passes close to the photoconductor surface, it pulls the toner from the photoconductor surface onto the paper still in the pattern of the image developed from the photoconductor surface.
After receiving the image, the paper is passed through a fuser nip. The fuser nip is the contacting area between a heating element and a pressure roller. The fusing process permanently adheres the image to the paper.
After being fused, the paper continues its path through further output driver rollers to exit the printer. The paper may, optionally, exit through face up output rollers or through further face down output rollers to an output tray.
Since the fusing process uses high heat and pressure to adhere the image, the paper becomes supple as moisture escapes. As such, or for other reasons, the paper will at times adhere to and begin to wrap around the heating element as the paper exits the fuser nip, thus causing a paper wrapping jam. To detect a wrapping jam, a fuser sensor flag is often used after the fuser nip and before the output rollers, to sense whether the paper has wrapped or is continuing its projected path.
However, the fuser sensor flag does not detect paper accordion jams in a timely manner. An accordion jam will eventually be detected in the event the fuser sensor is not timely released, but by that time the accordioned paper can be wedged in so tight that it may require a service call to clear it.
Accordion jams occur when too much drag or compressive force is applied to a leading area of the paper. This drag or force typically initiates, for example, at the output tray by a user interfering with the exiting process, or by too much paper being stacked in the output tray causing a flow block, or by any one of a number of other factors. Since the fuser nip continues to push/process the paper in a forward motion, and the paper is supple from fusing, when too much drag is applied to the leading edge area, the excessive drag or force causes a force wave to propel through the paper from the fuser nip toward the leading edge. As such, the wave causes the paper to fold in an accordion effect near the fuser nip, thereby creating an accordion jam.
Accordion jams are not detectable by a conventional fuser sensor flag in a timely manner because the paper remains in contact with the fuser sensor flag during processing, whether an accordion jam occurs or not. An accordion jam is especially undesirable because it can be difficult to clear. The accordioned portion of the paper is typically inaccessible near the fuser within the printer. However, if an according jam can be detected early on, error processing of the printing system can be improved to avoid harsh jams.
Accordingly, objects of the present invention are to provide an improved system and method for sensing according jams in a laser printer.